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BIG PHYSICS, BIG QUESTIONS –

Most distant solar system object yet could hint at hidden planet

Mikkel Juul Jensen/Science Photo Library

By Joshua Sokol

The inky black of the outer solar system just got a little brighter. A speck of light spotted in October 2015 is a rocky world more than 3 times more distant than Pluto – the farthest body in our solar system ever seen.

“We don’t know anything about its orbit,” says Scott Sheppard of the Carnegie Institute of Washington, whose team discovered the new addition. “We just know it’s the most distant object known.”

Sheppard announced the new object, called V774104, on 10 November at a meeting of the American Astronomical Society’s Division for Planetary Sciences, held in National Harbor, Maryland.

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From how it shifted in the sky as the Earth moved over a few hours, Sheppard’s team calculates that V774104 is about 103 astronomical units (AU) away from the sun, where one AU is the distance from Earth to the sun. That’s about as far away as the twin Pioneer probes, which have been traveling since 1972 and 1973.

To be as bright as it is at that distance, the object needs to be between 500 and 1000 kilometres in diameter, less than half the size of Pluto.

And it’s not alone. The same deep sky survey, conducted with the Subaru telescope in Hawaii and the Dark Energy Survey Camera in Chile, has also turned up about a dozen other objects around 80 to 90 AU from the sun. Since these distant bodies move around the sky slowly, it will take about a year of follow up observations to understand their orbits – and their origins.

If it turns out their paths will take them inward near Neptune’s orbit, they were probably kicked out of the inner solar system after a brush with Neptune. But Sheppard hopes that some will turn out to belong to a class of true weirdos: the inner Oort Cloud.

Dark planet

Only two other known objects are thought to be members of this exclusive club: Sedna, discovered in 2003, and 2012 VP113, found in 2012. Neither of them ever comes closer to the sun than 50 AU. The rest of the Oort Cloud, which is thought to be a storage lot for long-period comets, extends out a hundred or even a thousand times farther than these objects.

“Sedna and VP113 are the only object in the known solar system whose orbits cannot be explained by things in the known solar system,” says Michael Brown of the California Institute of Technology. They are far enough away from the giant planets to avoid gravitational tweaks to their orbits, and close enough to the sun that they don’t respond to other passing stars.

But the more likely explanation, according to Sheppard and Brown, is that the inner Oort Cloud preserves signs of an era at the very start of the solar system, when the sun and planets were born in an interstellar nursery packed tightly with nearby stars.

Over the next year, monitoring the new candidate objects may uncover a rare few that never venture into the inner solar system. If so, their orbits, added to Sedna and VP 113, should help us understand what influences such far-out worlds. Meanwhile, we have to wait to see what becomes of V774104.